It is known that when a nerve or the like is damaged it generates a pain signal. This current can be countered with an electrical current that is passed through the tooth.
Such a method has been shown to be effective for short periods of time, however after a period of such anesthesia, usually in the neighborhood of one minute, the pain-reducing effect of such a direct current being passed through the tooth decreases. To counteract this, it is known to alternate the current in one direction with a current of similar duration and amplitude in the opposite direction. The dentist or dental technician periodically throws a switch which reverses the polarity of the two probes in the patient's mouth and effects this alternation.
It has also been discovered that this latter system is relatively ineffective after long periods of time. It appears that the direction of current flow after predetermined periods of time becomes ineffective to eliminate nerve-generated pain signals. In addition such a system is relatively clumsy to operate, especially by a person who is often busy with more difficult tasks such as the filling of teeth or the like.